Great Falls, on the Potomac River at Great Falls Park
An early writer (in 1676) noted that the waterfalls of Virginia were not concentrated in just the mountains:
The state's waterfalls are concentrated in two areas, in the Blue Ridge and along the Fall Line ("sixty or seventy miles distant from the Mountains"). To make a waterfall, Mother Nature and Father Time need three ingredients:
The water part is easy in Virginia. On the average, over 40 inches of rain falls each year across the state. Some is absorbed by vegetation and evaporates (actually, "transpires") from leaves and needles. The cooling effect of evapotranspiration is what keeps temperatures from reaching above 100 degrees on most summer days in Virginia.2 Water that evaporates won't run downhill and over a waterfall, of course. The volume of Virginia waterfalls might be less impressive in August than in April, but few streams dry up completely in normal years.
The presence or absence of topographic relief determines where Virginia waterfalls are located, and the resistance of the rock determines the type of waterfall. Tidewater east of Interstate 95 is the flattest portion of the state, but there are still a few bluffs over 20' high - even on the Eastern Shore north of Kiptopeke State Park, for instance. In theory those bluffs would provide enough of a drop in elevation to create waterfalls, whever a stream crossed them.
However, to make a waterfall in Virginia, you also need bedrock that resists erosion. The bedrock of the Coastal Plain consists of weakly-consolidated sediments. Streams crossing the bluffs quickly etch a valley into the soft rock, rather than form an impressive waterfall.
At the Fall Line, all three ingredients are present for making waterfalls. Virginia rivers drop 50-80 feet in elevation from the Piedmont to sea level. The bedrock of the Piedmont (unlike the Coastal Plain) is hard, metamorphic rock resistant to erosion. The James River has eroded into the bedrock from the I-95 bridge upstream to the Huguenot Bridge, so the James River crosses the Fall Line in a series of rapids rather than in one giant Niagara-like waterfall.
The Potomac River has etched into the bedrock from the Teddy Roosevelt Bridge in the District of Columbia, carving a gorge upstream. The rock at the upstream end of the gorge is particularly resistant, so nearly all of the elevation drop occurs at Great Falls now. (Great Falls is 11 miles upstream from the contact between Piedmont and Coastal Plain, so the creation of the gorge demonstrates the power of the Potomac River to erode even the metamorphosed bedrock.)
Not all Virginia rivers cross the Fall Line, and not all riffles in a river are high enough to qualify as a "waterfall." Boaters on the downstream section of the Shenandoah River are well aware of the tiny limestone ledges that require some skill in "reading the water" to avoid turning over a canoe or running a bass boat aground. Bull Falls near Harpers Ferry is the most significant ledge on the Shenandoah, but the Shenandoah enters the Potomac River far upstream of the Fall Line at Great Falls.
The New River, the Big Sandy, the Holston, the Clinch, and the Powell as well as other streams on the western side of the Eastern Continental Divide flow to the Mississippi River, never coming near the Fall Line in Eastern Virginia. There are waterfalls along those rivers too. The New River flows through McCoy Falls (called "Big Falls" in the Delorme Atlas and Gazetteer) where it cuts through Walker/Sinking Creek Mountain downstream from Radford. Further upstream, where the New River drops over a small ledge, a textile factory was built to take advantage of the waterpower. That waterfall stimulated development of the town of Fries.
Virginia's tallest waterfalls are in the Blue Ridge, because there is plenty of water, hard bedrock - and elevation differences are greatest there. There is nearly 2,000' of topographic relief between the top of the mountains and the base of the Blue Ridge in the valley floor to the west, or the Piedmont to the east. The smaller streams in the mountains can drop spectacular distances, creating impressive scenic attractions in Shenandoah National Park and along the Blue Ridge Parkway. In the Blue Ridge, streams have not finished the physical process of eroding the hard volcanic or granitic bedrock and levelling the mountains into a flat plain.
